Amplification of laser light is required for a variety of applications. Long haul telecommunication applications, such as those employing single mode optical fiber, often require optical repeater/amplifiers to boost sagging signal levels. Material processing applications may require very high power laser light to perform functions such as cutting of various materials and preparation of material surfaces. Optical pumping of a solid state laser medium is a common and conventional method used to create a population inversion of energy states for laser applications requiring high-gain.
The laser medium providing this high-gain may comprise a material such as neodymium yttrium-aluminum garnet (Nd.sup.3+ :YAG) or Erbium doped optical fiber (Er.sup.3+ :silica), but it is well known in the art that any suitable material capable of maintaining an inverted population of energy states when optically-pumped will suffice. Those laser media utilizing Nd.sup.3+ :YAG are common, given the substantial optical gain near desired wavelengths near the 1.064 .mu.m range. Additionally, Nd.sup.3+ :YAG laser media provide linearity of pumping rate with respect to inverted population given its four-level transition system that is also well known to those skilled in the art.
To saturate an entire laser medium with an inverted population through optical pumping, a conventional method is to distribute a large array of laser diodes across the surface of the laser medium to form a pumping array. The light emitted from the individual laser diodes of the pumping array excites the laser medium and provide a very high optical gain for the energy transition level of the optically-pumped, inverted population within the high-gain laser medium, e.g., near the 1.064 .mu.m range for Nd.sup.3+ :YAG, near the 1.55 .mu.m range for Er.sup.3+ :silica, etc.
FIG. 1A (prior art) depicts a conventional, optically-pumped high-gain laser system 10. The pumping array shown in FIG. 1A is a one-dimensional array, but a two-dimensional array could also be used without departing from the scope and spirit of the invention. The conventional configuration comprises a laser medium 12, on which an array of laser diodes are distributed to form a pumping array 14. The pumping array is powered by a diode driver 16 which delivers electric current to the laser diodes of the pumping array to optically excite the laser medium 12 to produce the desired high optical gain; the light emitted from the pumping array 18 is, ideally, distributed throughout the entire laser medium 12. Different embodiments are used in the conventional system to generate the initial light beam that is to be amplified. FIG. 1A depicts a seed laser beam 20 being injected into the laser medium 12, which then undergoes amplification before exiting as an exit beam 22 for some desirable and useful purpose. As shown in FIG. 1B (prior art), the output optical power of the laser diodes 18 is proportional to the electric current delivered to the pumping array 14 above the lasing threshold of the laser diodes comprising the pumping array.
This conventional configuration delivers an electric current profile across the entire pumping array to create, ideally, a homogenous energy state population inversion throughout the laser medium. Typically, all the laser diodes in the pumping array receive exactly the same amount of electric current, resulting in a uniform current profile 21 across the pumping array (see FIG. 2A, prior art).
Often the pumping array is already integrated onto the laser medium with internal electric circuitry which partitions the current evenly among the various laser diodes of the pumping array 14, and the user must simply provide a single electric current from a diode driver 16. A typical electric circuit which would provide for such even electric current partition is shown in FIG. 2B (prior art). A voltage supply 26, capable of providing a voltage of nominally 2n V, is connected across the laser diodes of the pumping array 14, wherein n is the number of laser diodes within the pumping array. A switch 27 and a current limiting resistor 28 are connected in series with the laser diodes of the pumping array 14 to protect the pumping array from overcurrents and to provide on/off operation of the pumping array. A Zener diode 29 may be used to provide reverse voltage protection of the laser diodes of the pumping array. The breakdown knee voltage of the Zener diode 29 may be appropriately chosen to ensure that a reverse voltage is never applied across the terminals of the pumping array. This electrical configuration is merely exemplary of a method which may be used to deliver the conventional uniform current profile across all of the laser diodes of the pumping array 14.
While the conventional configuration is simplistic in its application and ease of operation in that a single electric current may be applied to the device, one resultant problem from employing this conventional method of a uniform electric current profile 21 across the entire pumping array 14 is from the steep and short thermal transition zone at the periphery 24 of the laser medium 12 (see FIG. 1A, prior art). This steep and short transition zone between the optically-pumped region and the un-pumped region results in a large thermal gradient near the periphery 24 of the laser medium.
This large thermal gradient near the periphery 24 of the laser medium 12, combined with the large thermal dependence of the refractive index of the laser medium 12 result in a distorted optical medium at the periphery 24. The exit beam 22 leaves the laser medium 12 at this thermally distorted periphery 24, resulting in wavefront distortion of the exit beam 22. These wavefront distortions are caused primarily from the temperature dependence of the index of refraction in the lasing medium. This phenomena often limits the total power that can be extracted from a lasing medium while maintaining a beam of acceptable quality.
The distortion of exit beam 22 may result in a poor quality spatial mode, unsuitable for coupling into a single mode optical fiber, resulting in either very lossy coupling or modal dispersion during transmission of the light beam within telecommunication applications.
The distortion of the exit beam 22 may also result in unstable beam pointing performance which is very undesirable for material processing applications where great accuracy is required for either precision cutting of specialty components or material surface preparation.
Additionally, for high frequency switching applications, high pulse rates may result in greater distortion of the exit beam 22. This effect may seriously limit the performance of the laser in terms of maximum output power while operating at high frequency pulse rates. An operating trade-off may result in which pulse rate and output power must be balanced.
Also, the amount of distortion of the exit beam is often non-linear with respect to increasing pulse rate. Without incorporating some additional compensatory system, a single, scaleable design capable of use in a variety of applications which may require widely varying pulse rates are virtually impossible to construct.
Systems which provide for launching light into a fiber at very high power levels and high pulse rates often incorporate a compensatory lens system to ensure focus of the exit beam 22 at high pulse rates, as the exit beam 22 distortion is greatly accentuated at high pulse rates. If the pulse rate is then decreased and the high power level is maintained, the frequency-induced exit beam 22 distortion is lessened, but the compensatory lens system continues to focus the exit beam which is of higher quality at lower pulse rates, thereby creating a very high exit beam 22 power density which can damage the optical fiber face.
In general, the wavefront distortions problems arise from the fact that the surface of the laser medium, when undergoing a very steep and short thermal gradient, results in a distorted medium through which the exit beam 22 must pass. This invariably results in distortion of the exiting wavefront which gives deleterious residual effects in a wide variety of applications.